Cyanine dye-sensitized photoconductive compositions comprising zinc oxide



prll 7, 1964 w. B. KENDALL ETAL 3,128,179

CYANINE DYE-SENSITIZED PHOTOCONDUCTIVE coMPosITIoNs coMPRsING ZINC oxInE Filed Sept. 2, 1960 f I llll mmm l llll llllllll llll lll. 300 400 500 600 700 my Fig. 2

w/LL/AM 5. KEA/DALL PAUL H. STEWART INVENTORS A TTORNEYS United States .Patent Office maire Patented Apr. 7, 1954 CYANINE DYE-SENSITIZED PHOTOCONDUCTIVE COMPOSITIONS COMPRISING ZINC OXIDE William BrKendall and Paul H. Stewart, Rochester, N.Y.,

assignors to Eastman Kodak Company, Rochester,

NLY., a corporation of New Jersey Filed Sept. 2, 1960, Ser. No. 53,689 Claims. (Cl. 96-1) This invention relates to optically sensitized photoconductive layers comprising zinc oxide which are particularly useful in making photographic copies (black and white, or color), and a method of making such photoconductive layers.

This application is a continuation-in-part of our application Serial No. 630,463, filed December 26, 1956, now abandoned.

It is known that zinc oxide can be employed in making photoconductive layers on ordinary paper and that photographic copies can conveniently be prepared from these photoconductive papers. This process has been described as being somewhat similar to the system known as xerograplly in 'that a photoconductive plate is employed and after exposure of the plate to a photographic image, development of the latent image is accomplished by means of a pigmented 'resinous composition which adheres to the unexposed portions of the exposed plate. However, the xerographic plate is generally used to transfer the developed image to a receiving sheet, Whereas the known system for using photoconductive zinc oxide generally makes use of the zinc oxide layer itself as a means of providing the desired photographic copy without transfer of any electrostatic charge to a receiving sheet.

In the known systemof employing zinc oxide in photoconductive layers, the grounded support, which is generally paper, is first made sensitive to light by giving it a blanket negative electrostatic charge on the zinc oxide layer in the substantial absence of any ultraviolet or visible radiation. One convenient means of giving the zinc oxide layer an electrostatic negative charge is by means of ion transfer from a corona discharge. The zinc oxide photoconductive layer can then be exposed to a photographic image in the usual manner, the portions of the zinc oxide which receive light or ultraviolet radiation losing wholly, or in part (depending upon extent of exposure), the negative electrostatic charge, while the unexposed portions of the photoconductive layer retain their negative electrostatic charge. The resulting latent image can then be developed by means of a pigmented resin powder which has a charge opposite to the negative charge of the unexposed areas of the photoconductive layer. The pigmented powder is thus rmly attached or attracted to the negatively charged areas. The pigmented resin powder can then be axed to the photoconductive layer by simply melting the resinous vehicle at a temperature below the charring temperature of the paper support, so that the resinous powder becomes fused to the surface of the'original photoconductive layer. Various means of developing the latent image in the photoconductive layer to a visible image have been described in the prior art.

One disadvantage in the zinc oxide normally used in such photoconductive layers is that the light-sensitivity tized according to our invention.

'of such charged zinc oxide normally is at its greatest in the ultra-violet region of the spectrum, whereas the exposing source 'may have its maximum output in a region of the spectrum which lies within the visible region, such as an ordinary tungsten light. While various means have been previously described for sensitizing the zinc oxide so that it has some panchromatic or orthochromatic sensitivity, lfor example, by means of various organic dyes such as Rose Bengal, and the like, these methods have not been particularly satisfactory since the disadvantage of strongly dyeing the zinc oxide layer more than offsets thesensitivity which is supplied to the feebly sensitive zinc oxide. That is, it is generally desirable to have some rmeans of sensitizing the zinc oxide which does not permanently color the zinc oxide layer, which might be used as the final copy of the photographic image. Strong coloration of the zinc oxide layer has an unfavorable aesthetic effect and might be strongly objectionable in the event that it is desired to make color prints of the original subject. Other unfavorable effects, such as poor contrast, slow speeds, etc., are evident.

It is an object of our invention to provide a convenient means of optically sensitizing zinc oxide photoconductive layers in a useful manner. Another object is to provide a means of optically sensitizing zinc oxide photoconductive layers by means of certain acidic cyanine dyes. Still another object is to provide a particular class of cyanine vdyes which are outstanding in their optically sensitizing properties for zinc oxide photoconductive layers. Other objects willbecome apparent from a consideration of the following description and examples.

Our invention is illustrated graphically in the accompanying drawing where FIGURES 1 and 2 are spectrograms of photoconductive zinc oxide compositions sensi- It is to be understood that by the term photoconductive zinc oxide compositions, we mean zinc oxide compositions commonly used in the field of electrophotography which are characterized by photoconductive properties. The zinc oxide frequently employed in such compositions is known as French process zinc oxide.

The particular class of cyanine or polymethine dyes which can be used in practicing our invention comprise dyes known in the art as simple cyanine dyes, carbocyanine dyes and dicarbocyanine dyes, provided such dyes contain at least one carboxyl group. The carboxyl group can be attached directly to the molecule of the parent or carboxyl-free dyes, or the carboxyl group can be attached to the molecule through an alkyl or aryl group. Some of the dyes useful in our invention do not contain the conventional acid anion, which is normally associated with one of the basic nuclei, these dyes normally being characterized as anhydronium bases. It is to be understood that our invention contemplates not only the dye salts as hereinafter identified, but that our invention contemplates the corresponding anhydronium bases which do not contain the aforementioned characteristic acid anion. The cyanine or polymethine dyes useful in practicing our invention can be represented by the following general formula:

wherein R and R1 each represents an alkyl group (e.g., methyl, ethyl, n-propyl, n-butyl, isobutyl, n-amyl, isoamyl, etc), a hydroxy alkyl group (e.g., -hydroxyethyl, fy-hydroxypropyl, etc.), an alkoxyalkyl group (e.g., -methoxyethyl, -ethoxyethyl, etc.), a carboxyalkyl group (e.g., carboxymethyl, -carboxyethyl, a-carboxyethyl, -carboxybutyl, fy-carboxypropyl, etc.), a sulfoalkyl group (e.g., sulfomethyl, IB-sulfoethyl, y-sulfopropyl, -sulfobutyl, etc), a carbalkoxyalkyl group (e.g., carbomethoxymethyl, -carbomethoxyethyl, carbethoxyethyl, -carbethoxyethyl), an acyloxyalkyl group (e.g., -acetoxyethyl, y-acetoxypropyl, etc), an aralkyl group (e.g., benzyl, -phenethyl, etc.), etc., R2 represents a hydrogen atom, an alkyl group, which can bear substituents (e.g., methyl, ethyl, n-propyl, carboxymethyl, -carboxyethyl, etc.), or an aryl group, which can bear substituents (e.g., phenyl, m-, and p-carboxyphenyl, o, mand p-hydroxyphenyl, etc), m and n each represents a positive integer of from 1 to 2, q represents a positive integer of from l to 3 (provided R2 represents a hydrogen atom when q is 3), X represents an acid anion (e.g., chloride, bromide, iodide, thiocyanate, methylsulfate, ethylsulfate, benezenesulfonate, p-toluenesulfonate, perchloride, acetate, etc), and Z and Z1 each represents the non-metallic atoms necessary to complete a heterocyclic nucleus containing from (m and n, respectively, being 1) to 6 atoms in the heterocyclic ring.

Typical heterocyclic nuclei dened by Z and Z above include, for example, a thiazole nucleus (e.g., thiazole, 4 methylthiazole, 5 methylthiazole, 4 phenylthiazole, 5-phenyltl1iazole, 4,5-dimethylthiazole, 4,5-diphenylthiazole, 4(2-thienyl)thiazole, etc.), a benzothiazole nucleus (e.g., benzothiazole, 4-chlorobenzothiazole, S-chlorobenzothiazole, 6-chlorobenzothiazole, 7-chlorobenzothiazole, 4 methylbenzothiazole, 5 methylbenzothiazole, -rnethylbenzothiazole, 5bromobenzothiazole, 6-bromobenzothiazole, 4-phenylbenzothiazole, 5-phenylbenzothiazole, 4-methoxybenzothiazole, S-methoxybenzothiazole, 6 methoxybenzothiazole, 5 iodobenzothiazole, 6 -iodobenzothiazole, 4 ethoxybenzothiazole, 5 ethoxybenzothiazole, tetrahydrobenzothiazole, 5,6 dimethoxybenzothiazole, 5,6 dioxymethylenebenzothiazole, 5 hydroxybenzothiazole, 6 hydroxybenzothiazole, 5 carboxybenzothiazole, etc.), a naphthothiazole nucleus (e.g., a-naphthothiazole, -naphthothiazole, S-methoxy--naphthothiazole, 5 ethoxy naphthothiazole, 7methoxya naphthothiazole, 8-methoxy-a-naphthothiazole, etc.), a thianaphtheno-7,6',4,S-thiazole nucleus (e.g., 4-methoxy thianaphtheno-7,6,4,5thiazole, etc.), an oxazole nucleus (e.g., 4-methyloxazole, S-methyloxazole, 4-phenyloxazole, 4,5-diphenyloxazole, 4-ethyloxazole, 4,5-dimethyloxazole, S-phenyloxazole, etc.), a benzoxazole nucleus (e.g., benzoxazole, 5 chlorobenzoxazole, 5 phenyloenzoxazole, S-methylbenzoxazole, 6-methylbenzoxazole, 5,6-dimethylbenzoxazole, 4,6-dimethylbenzoxazole, S-methoxybenzoxazole, 6 methoxybenzoxazole, 5 ethoxybenzoxazole, 6-chlorobenzoxazole, S-hydroxybenzoxazole, 6-hydroxybenzoxazole, 5-carboxybenzoxazole, etc.), a naphthoxazole nucleus (e.g., :x-naphthoxazole, ,B-naphthoxazole, eten), a selenazole nucleus (e.g., 4-methylselenazole, 4-phenylselenazole, etc), a benzoselenazole nucleus (e.g., benzoselenazole, 5chlorobenzoselenazole, S-methoxybenzoselenazole, S-hydroxybenzoselenazole, tetrahydrobenzoselenazole, etc.), a naphthoselenazole nucleus (e.g., a-naphthoselenazole, -naphthoselenazole, etc.), a thiazoline nucleus (e.g., thiazoline, 4-methy1thiazoline, etc.), a 2-quinoline nucleus (e.g., quinoline, 3-methylquinoline, S-methylquinoline, 7-methylquinoline, S-methylquinoline, -chloroquinoline, 8 chloroquinoline, 6 methoxyquinoline, -ethoxyquinoline, 6-hydroxyquinoline, S-hydroxyquinoline, etc.), a 4-quinoline nucleus (eg, quinoline, 6-methoxyquinoline, 7-methylquinoline, S-methylquinoline, ete), a l-isoquinoline nucleus (e.g., isoquinoline, 3,4-dihyd1oisoquinoline, ete), a 3,3-dialkylindolenine nucleus (e.g., 3,3-dimethylindolenine, 3,3,5-trirnethylindolenine, 3,3,7-trimethylindolenine, etc.), a Z-pyridine nucleus (e.g., pyridine, S-methylpyridine, 4-methylpyrdine, 5-methylpyridine, -methylpyridine, 3,4-dimethylpyridine, 3,5-dimethylpyridine, 3,6-dimethylpyridine, 4,5-dimethylpyridine, 4,6-dirnethylpyridine, 4-chloropyridine, 5-chloropyridine, 6-chloropyridine, 3-hydroxypyridine, 4-hydroxypyridine, 5 -hydroxypyridine, -hydroxypyridine, 3 phenylpyridine, 4 phenylpyridine, 6 phenylpyridine, etc.), a 4- pyridine nucleus (e.g., 2 -methylpyridine, 3-methylpyridine, 2-chloropyridine, 3-chloropyridine, 2,3- dimethylpyridine, 2,5-dimethylpyridine, 2,6-dimethylpyridine, Z-hydroxypyridine, 3-hydroxypy1idine, etc.), etc.

As indicated above, the cyanine or polymethine dyes of our invention having a carboxyl group attached to the nitrogen atom of the Z or Z1 nucleus through an alkyl group sometimes exists in the form known as anhydronium bases; that is these dyes do not separate from their preparative medium in the form of conventional quaternary salts (X represents an acid anion), but in the form of compounds which can be regarded as derived from the compounds of Formula I above by removing the X radical from the formula and placing a positive charge on the nitrogen atom of the thus modified basic nucleus. These compounds are called anhydronium bases since they are regarded as derived from a quaternary ammonium hydroxide corresponding to the compounds of Formula I, except that X would be regarded as a hydroxyl group. Removal of the atoms of water from the hydrogen of the carboxyl group which may be connected to the nitrogen atom of the basic nucleus through an alkyl group and the hydroxyl group which is regarded as being attached to the nitrogen atom of the second basic nucleus directly, gives a structure which corresponds to the anhydronium bases Well known to those skilled in the art of sensitizing dyes.

The carboxyl-substituted dyes of our invention are markedly superior to the carboxyl-free dyes which are also well known to those skilled in the art of spectral sensitizing dyes.

Cyanine or polymethine dyes useful in practicing our invention include the following.

( 1) Anhydro3,3-di-earboxyethyl-5,5'-dichloro-9- methylthiacar-bocyanine hydroxide (2) Anhydro-3,3'-dicarboxymethyl-S,5-dichlorothiacar bocyanine hydroxide (3) 3--carboxyethyl-3,9-diethyl-5,5 '-dichflorothiacarbocyanine iodide (4) 3-(4-carboxybutyl)-5,5'-d'ichloro-9-ethyl-3- methylthiacarbocyanine iodide (5) 3-carboxyethyl3'-ethyl-9-methylthiacarbocyanine iodide (7) Anhydro-3,3-di--carboxyethyloxacarbocyanine hydroxide (8) 5,5-dicarboxy3,3dimethyloxacarbocyanine-ptoluenesulfonate (9) 3,3diethyl-9-o-carboxyphenyl-4,5,4,5dibenzothia carbocyanine-fp-toluenesulfonate 11) 1--carboxyethyl-1'-ethyl-3,3-dimethylindo-2- carbocyanine iodide 12) Anhydro-1,1'-dicarboxymethyl-4,4'-carbocyanine hydroxide (13) 9-canboxy-l,1-diphenyl-2,2carbocyanine iodide 14) 3,3di(carboxymethyl)oxathiadicarbocyanine bromide 15) 3,3-diethyl-9-o-earboxyphenylthiacarbocyanine iodide 16) 3,3-diethyl-7-o-carboxyphenylthiazolinocarbocyanine iodide 17 Anhydro-S',3'-di--carboxyethyl-Sl-ethyloxacarbocyanine hydroxide 18) Anhydro-3,3 dicarboxyrnethyl-5,5 '-diphenyloxacarbocyanine hydroxide (20) 3-'carboxymethyl-1-ethyl-6-methoxythia-2'-cyanine bromide (24) Arrhydro-S--carboxyethyl-S,5'-dichloro9-ethyl 3'--sulfoethylthiacarbocyanine hydroxide (25) Anhydro-3,3'-di-,B-.carboxye-thylthiadicarbocyanine hydroxide (26) 3'-carboxymethyl-3-ethyloxathiadicarbocyanine iodide (27) 3-carboxyrnethyl-3'-ethyloxathiadicarbocyanine iodide (28 3,3'di (carboxymethyl) oxathadicar-bocyanine bromide Method for making simple cyanine `dyes useful in practicing our invention have been previously described in the prior art. Among the patents describing methods suitable for preparing such dyes are the following:

United States 1,935,696, issued on November 21, 1933 United States 2,108,485, issued on February 15, 1938 United States 2,120,322, issued on June 14, 1938 British 424,559, issued on February 18, 1935 British 504,821, issued on April 27, 1939 Methods for making carbocyanine dyes useful in pr-acticing our invention have likewise been described in .the prior art. Among the patents describing methods suitable for preparing such dyes are the following:

United States 1,934,657, issued on November 7, 1933 United States 1,934,659, issued on November 7, 1933 United States 1,950,876, issued on March 13, 1934 United States 1,969,444, issued on August 7, 19,34 United States 1,994,563, issued on March 19, 1935 United States 2,072,908, issued on March 9, 1937 United States 2,107,379, issued on February 8, 1938 United States 2,213,238, issued on September 3, 1940 United States 2,231,658, issued on February 11, 1941 United States 2,233,509, issued on March 4, 1941 United States 2,503,776, issued on April 11, 1 950 United States 2,609,371, issued on September 2, 1952 British 528,574, accepted November 1, 1940 As shown above in dye 13, for example, the dyes of our invention can have a carboxyl group attached to the intercyclic polymethine chain. Such dyes containing a quinoline nucleus can be prepared according .to the Wellknown technique of reacting glutaconic acid with an alkylmercaptoquinolinium salt. Cyanine dyes of Formula I wherein Z and Z1 represent the non-metallic atoms necessary to complete a quinoline nucleus, respectively, can also Ibe used in our invention in cases wherein R and R1, respectively, representan aryl group, such as phenyl, tolyl, etc. The following example will serve to illustrate the preparation of such a dye.

COOH I A mixture of 2amethylmercapto-l-phenylquinolinium p-toluenesulfonate (8.46 g., 1 mok), glutaconic acid (2.6

g., 1 mol.+l00%) and triethylarnine (2.02 g., 1 mol.-1100%) was dissolved in pyridine (30 ml.) and heated under reflux for 30 minutes. The crude dye was precipitated with sodium iodide (3 g. in 10 ml. H2O) and the mixture extracted with several ml. portions of ether. After decanting the ether, the crude dye was collected on a filter, rwashed with Water and dried. After three recrystallizations from methyl alcohol, the yield of purified dye was 0.55 g. (10%), M.P. 214-216 C. dec.

Methods for making disca-.ubocyanine dyes useful in practicing our invention have also been described in the prior art. Among patents describing methods suitable for preparing such dyes are the following:

United States 2,213,995, issued on September 10, 1940 United States 2,238,231, issued on April 15, 1941 British 354,826, issued on August 17, 1931 'Ihe above optical sensitizing dyes can be combined with the zinc oxide photoconductive material in any convenient manner. For example, the optical sensitizing dye can be added to ythe zinc oxide composition while dissolved in water or an organic solvent. Pyridine, methanol, ethanol, acetone, water, etc., can be used to dissolve many of the cyanine dyes useful in practicing our invention. The zinc oxide can be uniformly dispersed in an organic solution of the binder customarily employed for the zinc oxide and a solution of the polymethine or cyanine dye added to this coating solution. After thorough mixing, the sensitized solution can be coated on a paper support and dried in the usual manner.

Alternatively, an unsensitized zinc oxide coating can be prepared as described above and after removal of the organic solvent, the paper coating can be immersed in a solution (organic or aqueous) of the cyanine dye. This method has been found to be particularly useful in that higher speeds can be frequently obtained.

The binders for the zinc oxide comprise many of the resinous compositions which are commercially available. Such resins are sold under trade names, such as Plaskon ST y856, Rezyl 405-18, Pliolite S-7, Styresol 4440, DC 804, etc. These resins comprise ystyrene-butadiene copolymers, silicone resins, styrene-alkyd resins, siliconeal'kyd resins, soya-alkyd resins, polyvinyl chloride, polyvinyl acetate, etc. The methods of making such resins have been previously described in 4the prior art. IFor example, ystyrene-alkyd resins can be prepared according 'to the method described in U.S. Patent 2,631,019', issued Olctober 24, 1944; U.S. Patent 2,258,423, issued October 7, 1941; U.S. Patent 2,453,665, issued November 9, 1948, etc. Other binders, such as parain, mineral waxes, etc., can also be employed. These binders are generally characterized as having marked hydrophobic properties (i.e., being substantially -free of any Water-solubilizing groups, such as hydroxyl, free acid groups, amide groups, etc.) and as being good electrical insulators or as having high electrical resistivity. These binders can be easily dissolved in organic solvents having a boiling point below the charring temperature of the paper support. Also, these binders have the desirable property of readily dispersing the zinc oxide photoconductive material. Some resinous ybinders are relatively poor insulators and do not provide coatings which can be ystored for prolonged periods of times after the photoconductive coatings have been negatively charged. This is particularly noticeable at relatively high humidities, and the photoconductive coatings should be negatively charged lshortly before use in such instances, that i-s, it is not advisable to charge the photoconductive coatings too long in advance before use. Such problems are -well understood by those skilled in the art.

Nonpolar solvents have been found to be particularly useful in preparing the photoconductive layers in that any residual solvent which cannot be removed does vnot have a deleterious effect on the keeping qualities of the photoconductive layers. Such solvents include the aromatic hydrocarbons, such as benzene, xylenes, toluene, etc.

The zinc oxide photoconductive material employed in our invention should generally consist of relatively small particles of less than 0.5 micron mean diameter. Such zinc oxide materials are readily available and can be purchased under a variety of trade names, such as Protox No. 168 (New Jersey Zinc Company), etc. Sucient binder should be employed to insulate each of the zinc oxide particles from the surrounding particles in the composition. The most useful or optimum quantity of Zinc oxide to binder for a particular binder can be readily determined by making a series of test coatings wherein the quantity and relative amounts of zinc oxide to binder are employed.

Exposure of the charged photoconductive layer to Visible radiation or ultraviolet radiation causes a loss or reduction of the negative charge in those port-ions of the photoconductive material which are exposed to the radiation. The degree of loss will depend on the intensity and time of exposure -to the radiation, in general. The resulting latent electrophotographic image can then be developed to a visible image in a variety of rways, including those which have been previously employed in electrophotographic processes such as xerography. A particularly useful means of developing the latent electrophotographic image comprises use of a magnetic brush. This magnetic brush development makes use of a ferromagnetic powder, such as iron filings, which has been intimately mixed with pigmented resin, or sulfur. Agitation of the ferromagnetic powder and pigmented resin results in a triboelectric effect wherein the pigmented resin acquires an elec- -tric charge depending upon the relative position of the resin to the ferromagnetic powder in the triboelectric series. That is, ordinary iron powder is below most resins in the triboelectric series, and mixture with a resin higher in the `series results in the deposition of a positive electrostatic charge on the resin. The resulting mixture can then be picked up by a magnet on which the iron particles, or other ferromagnetic powder, arrange themselves in the conventional pattern, so that the long chains of filings resemble an ordinary brush. This magnetic brush can then be placed in contact with the exposed photo-conductive layer `and the brush passed across the negative electrostatic latent image which is on the surface of the photoconductive material. As the magnetic brush passes over the areas of the photoconductive material which have residual negative charge thereon, the electrostatic attraction between the charged pigmented resin particles and the oppositely charged image areas in the photoconductive material is greater than the attraction between these particles and the ferromagnetic powder, so that the pigmented resin is deposited on the surface of the photoconductive material roughly in proportion to the residual charge on the surface of the photoconductive layer. By selecting a resin with a low melting point, the developed image can then be xed to the surface of the paper by heating to a temperature above the melting point of the resin, but below the charring temperature of the paper. The resin in the pigmented resin compositions can -be varied, depending upon the effects desired and the type of copy which is being reproduced. Such resins may be the ysame as those employed in the insulating layer coated on the paper support, such as styrene-butadiene resins, etc. The particle size of the pigmented resin used in development can vary, although the range of 6.1 to 25 microns is adequate for most purposes. Various pigments can be used in the resin developing compositions. The ability of the pigmented resin to accept a positive charge is dependent upon the type of resin selected. The pigment merely serves to impart color to the resin and probably imparts very little, if any, influence on the overall charge of the pigmented resin.

The `following example will serve to illustrate the manner of using the optically sensitized photoconductive materials of our invention.

Example y parts by weight of zinc oxide was mixed with 36.4 parts of a silicone-alkyd resin (Plaskon ST 856, Allied Chemical and Dye Corporation) containing 55% solids, and the mixture added to 36.4 parts of xylene solvent and intimately mixed in a Waring Blender for `30 minutes. -An additional 67 parts by weight of xylene solvent were added to the resulting zinc oxide dope and the mixture coated on a glossy, single-weight baryta coated paper stock at a coverage of approximately 5 g./ft.2. A strip of the coated paper was then dipped for about 5 seconds in a 0.\1% by lweight methanol (or any other suitable solvent, such as those mentioned above) solution of B--carboxyethyl-1'-ethyl-6methoxy5-phenylthia2cyanine iodide (dye 2.2) containing a trace of triethylamine to facilitate dissolving of the dye. The sensitized strip was then removed from the solution and air dried in vertical position. After drying, the strip was charged under a corona discharge and exposed `for one-half second in asensitorneter to tungsten illumination. The exposed coating was then developed by the magnetic brush technique described above using small iron particles and black pigmented sulfur. Finally, the image was xed by fusing the black pigmented ysulfur to the paper surface by applying heat. A duplicate strip treated in the same manner, with the sensitizing dye being omitted, served as a control.

The electrical characteristics of the control and the dye-treated strips with respect to initial charge and dark decay were essentially the same. However, the speed of the dye-treated strip was 16 times that of the check strip containing no sensitizing dye.

In a similar manner, other optical sensitizing dyes of the cyanine dye series can be employed in our invention. In the following table are listed the results obtained by replacing the dye in the above example by other dyes of the cyanine dye series as identied above.

Percent Dye No. Concentration Relative W hlte of dye solution Light Speed (by weight) In a manner similar to that described in the above examples, photoconductive Zinc oxide (French process) was mixed with an insulating binder-material as described above (Plaskon ST 856, Allied Chemical and Dye Corporation) and coated on a conventional paper support. The paper support was then cut into a number of stirps and certain of the strips dipped into a suitable solution containing one of the optical sensitizing dyes in a concentration shown in the following table. The strips were then dried and exposed in a sensitometer to tungsten illumination. The exposed coatings were then developed by the magnetic brush technique described above.

The dye numbers given in the following tabulation correspond to those given above, although two dyes have been added in order to show that dyes having no freecarboxyl group have much less useful properties than then-dyes of the present invention. Dye (a) is 3,3,9 triethyl-S,5'-dichlorothiacarbocyanine bromide. Dye (b) is 3,3'-diethyloxacarbocyanine iodide. Of course, many of the simple or monomethine cyanine dyes useful in practicing our invention have their maximum absorption in the blue region of the spectrum. For this reason, it is apparent that the printing materials of our invention which have been spectrally sensitized with simple or monomethine cyanine dyes would optimally be exposed to an illuminating source which has a substantial emission in this region of the spectrum.

TABLE A Solvent and Percent Conc.

Dye

C ontrol 1 NnthLEtoH, .002 PyridmeA-Eton, .o1 Eton, .o1

Nuts-tunen, .o1 NnuHzoEtoH, .o1 Et .o1

Nm3-tutori,

The optical sensitizing dyes used in our invention can be employed in the manufacture of natural three-color photographic images. This process can be outlined as follows:

A panchromatically-sensitized electrophotographic paper of the type described in the above example was negatively charged under a corona discharge, exposed by projection through a positive color transparency and a blue filter to a known daylight quality light source for a definite time at a given distance and then developed by the magnetic brush technique described above using iron filings and a yellow pigmented resin, such as Hansa Yellow G pigment, dispersed in Pliolite S-5D. The yellow image corresponding to green, red and black areas of the positive transparency was then transferred electrostatically to the surface of an unsensitized paper receiving sheet and xed by fusion to the paper surface by heat. Using the same electrophotosensitive paper, after lightly brushing or blowing with air to remove any remaining developer, the same procedure described above was repeated using a green filter and a magenta developer, such as Roberts process red pigment dispersed in Pliolite S-5D. The magenta image corresponding to the blue, red and black areas of the positive transparency was transferred electrostatically in register to the surface of the same unsensitized receiving sheet containing the yellow image. The magenta image was then iixed by means of heat. Again, the procedure was repeated using a red lter and a cyan developer, such as peacock blue pigment dispersed in Pliolite S-SD. The resulting cyan image corresponding to the blue, green and black areas of the positive transparency was electrostatically transferred in register to the unsensitized receiving sheet containing the yellow and magenta images Vand xed by means of heat. The resulting print containing the fused yellow, magenta and cyan transferred images represented a -natural three-color reproduction of the positive transparency. Alternatively, instead of fixing each transferred image by heat fusion, the transferred images can be xed by spraying with a clear resin which hardens upon evaporation of the resin solvent. Also, the above procedure can be modified by using separate sheets of electrophotographic paper which have been optically sensitized to correspond to the spectral transmission range for the three-color separation filters mentioned above.

In a typical process run, Aa panchromatically-sensitized electrophotographic paper of the type described above containing a mixture of 3--carboxyethyl-2(3,3-dicyanoallylidene)benzothiazoline, anhydro 3,3 carboxyethyl-S,5-dichloro-9-ethylthiacarbocyanine hydroxide and 3,3-di hydroxyethylthiadicarbocyanine bromide as blue, green and red optical sensitizers, respectively, was exposed by projection through a positive Kodachrome transparency and a Wratten No. 47 filter (i.e., a filter transmitting light only between about 370 and 515 mit) for forty seconds. The exposed paper was developed by the magnetic brush technique described above and developed with a yellow pigmented resin of the type mentioned above. The yellow image was then transferred to a White clay-coated paper and fixed by fusion to the paper surface via means of heat. Using the same piece of electrophotosensitive paper, after lightly blowing with air to remove any remaining developer, the same procedure described above was repeated using an exposure time of forty seconds, a Wratten No. 61 lter (i.e., a lter transmitting light only between about 475 and 605 mp.) and a magenta pigmented resin of the type described above. The magenta image was then transferred in register to the surface of the receiving sheet and fixed by means of heat. Again, the same procedure was repeated using a Wratten No. 29 filter (i.e., a filter transmitting only light beyond 600 mfr) with an exposure time of four and one-half seconds and with development by means of a cyan pigmented resin. Finally, the cyan image was transferred electrostatically in register to the receiving sheet and fixed by means of heat. The resulting print was a three-color reproduction of the original Kodachrome transparency. The above example was repeated, except that the pigmented resin was not anchored to the paper surface by heat fusion, but by spraying with Krylon (an acrylic spray manufactured by Krylon, Incorporated, Philadelphia, Pennsylvania) after each transfer step. The Pliolite S-SD used above as a vehicle for the developing pigment is a styrene-butadiene copolymer and was chosen because of its frangibility, its positive position relative to iron in a triboelectric series and its ability to become fused to paper at a temperature below C. (the scorch temperature of paper). This resin was milled in a one-quart mill with liint pebbles for 214 hours and then sieved to remove all particles coarser than 200 mesh. The resin was then combined with the desired pigment and ground with mortar and pestle for 10 minutes. To minimize the background in non-transfer processes (as contrasted with the electrostatic transfer mentioned above) it is necessary to fuse the pigment and resin together, crushv the mixture, and regrind. To obtain very finely divided toners, the mixture can be passed through a micronizer of the type described in Haloid U.S. Patent No. 2,659,670, issued November 17, 1953.

It has been found that in some instances, the sensitizing action of certain of the dyes useful in our invention can be improved by heating the sensitized printing papers for a short period of time prior to charging and exposure. A short period of heat treatment appears to increase the etiiciency by which the sensitizing dye is able to transfer energy to the photoconductive zinc oxide.

The accompanying drawing illustrates schematically the increased spectral range of sensitivity provided by two of the dyes useful in practicing our invention. In FIGURES l and 2, the solid curves show the range of sensitivity as well as the region of maximum sensitivity. Exposures were made to daylight quality radiation in a spectrograph in the normal manner.

-InFIGURl-E 1, the solid curve represents the sensitivity of a photoconductive element comprising a relatively thin layer of photoconductive zinc oxide sensitized with anhydro-3,3di ,6' carboxyethyl 5,5 dichloro 9 ethylthiacarbocyanine hydroxide. The relative speed of this coating is shown in Table A above, wherein the dye is identified as dye 1.

In FIGURE 2, the solid curve represents the sensitivity of a photoconductive element comprising a relatively thin layer of photoconductive zinc oxide sensitized with 3-,5- carboxyethyl 1 ethyl 6 methoxy 5 phenylthia- 2'cyanine iodide. The relative speed of this coating is given in the example above, where the dye is identified as dye 22.

It has been found that the dyes of our invention containing a free-carboxyl group have markedly improved sensitizing action toward photoconductive zinc oxide as compared With the corresponding dyes containing no freecarboxyl group. This appears to be true regardless of the manner by which the carboxyl group is attached to the molecule of the parent sensitizing dye. In the compositions of the present invention, the carboxyl-containing dyes are characterized consistently by increased speeds as compared with dyes containing no free-carboxyl groups. By free-carboxyl group, it is to be understood that We mean not only -COOH, but water-soluble salts thereof (e.g., triethylammoniurn, sodium, pyridine, triethanol ammonium, etc.).

The invention has been described in detail with particular reference to preferred embodiments thereof, but it Will be understood that variations and modiiications can be effected Within the spirit and scope of the invention as described hereinabove and as defined in the appended claims.

What we claim as Iour invention and desire secured by Letters Patent of the United States is:

1. A photoconductive composition comprising photoconductive zinc oxide, a high dielectric insulator binder for said zinc oxide, and adsorbed to the surface 'of said zinc oxide, a cyanine dye containing la carboxyl radical attached to a carbon atom thereof.

2. A photoconductive composition comprising photoconductive zinc oxide, a high dielectric insulator binder for said zinc oxide, iand adsorbed to the surface of said zinc oxide a cyanine dye containing a carboxyalkyl group attached to the nitrogen atom of a heterocyclic nucleus thereof.

3. A photoconductive composition comprising photoconductive zinc oxide, a high dielectric insulator binder for said zinc oxide, yand in contiguity with slaid zinc oxide, a cyanine dye selected from those represented by the following general formula:

wherein Rand R1 each represents a member selected from the class consisting of an alkyl group, a hydroxyalkyl group, an alkoxyalkyl group, la carboxyalkyl group, a sulfoalkyl group, a carbalkoxyalkyl Igroup, an acyloxyalkyl group, and an aralkyl group, 4R2 represents a member selected from the :class lconsisting of la hydrogen atom, an alkyl group, and an aryl group, m yand n each represents a Ipositive integer of from 1 to 2, q represents -a positive integer of trom l to 3, X represents an 'acid anion, and Z and Z1 each represents the non-metallic atoms necessary to complete a heterocyclic nucleus selected from the cli-ass consisting of a thiazo'le nucleus, a benzothiazole nucleus, `a naphthothiazole nucleus, a thianaphtheno-7, 6,4,5thiazole nucleus, an ozazole nucleus, a benzoxazole nucleus, a naphthoxazole nucleus, a selenazole nucleus, a benzoselenazole nucleus, a naphthoselenazole nucleus, a thiazoliue nucleus, a Z-quinoline nucleus, a 4-quinol-ine nucleus, a 1-isoquinoline nucleus, a 3,3-di-alykylindolenine nucleus, a 2-pyridine nucleus and -a 4-pyridine nucleus, provided that (1) R2 represents a hydrogen atom` when q is 3, (2) m `and n each represents 1 when Z and Z1, respective'ly, represent the 'atoms necessary to complete a heterocyclic nucleus containing 5 atoms in the heterocyclic ring, and (3) said cyanine dye contains a carboxyl radical on at least one of the groups R, R1, R2, Z and Z1.

4. A photoconductive composition -as defined in claim 3 Wbherein the cy-anine dye in contguity with said photoconductive zinc oxide is an |anhydroniurn hydroxide of the dyes defined in claim 3.

5. A photoconductive `composition comprising photoconductive zinc oxide, a high dielectric insulator binder for said photoconductive zinc oxide, and in contiguity with said photoconductive zinc oxide anhydro-3,3di-- l2. carboxyethyl-5,5'-dichloro-9-methylthiacarbocyanine hydroxide.

6. A photocond-uctive composition compri-sing photoconductive zinc oxide, a high dielectric insulator binder for said photoconductive zinc oxide, and in contiguity with said photocouductive zinc oxide ianhydro3,3'dicarboxyethyloxacarbocyanine hydroxide.

7. A photoconductive composition comprising photoconductive zinc oxide, a high dielectric insulator binder for said photoconductive zinc oxide, and in contiguity with said photoconductive zinc oxide 3-(4carboxybutyl) 5,5 '-dichloro-9-ethyl-3'-methylthiacarbocyanine iodide.

8. A photoconductive composition comprising photoconductive zinc oxide, a high dielectric insulator binder for said photoconductive zinc oxide, and in contiguity with said photoconductive zinc oxide 1--carboxyethyl-1ethyl 3,3-dinrethylindo-Z-carbocyanine iodide.

9. A photoconductive composition comprising photoconductive zinc oxide, ta. high dielectric insulator binder for said photoconductive zinc oxide, and in contiguity with said photoconductive zinc oxide 3,3-di(carboxymethyl) oxathiadicarbocyanine bromide.

10. A photographic element for electrophotography comprising a paper support and a relatively thin layer comprising photoconductive zinc oxide, an organic, hydrophobic, a high dielectric insulator binder for said photoconductive zinc oxide and in contigui-ty with said photoconductive zinc oxide, a cyanine dye selected from those represented by the following general formula:

wherein R and R1 each represents a member selected from the class consisting of an alkyl group, a hydroxyalkyl group, an alkoxyalkyl group, a carboxyalkyl group, a sulfo-alkyl group, a car-balkoxyalkyl group, an iacyloxyalkyl group, and an aralkyl group, R2 represents a member selected from the class consisting of la hydrogen atom, an alkyl group, yand |an laryl group, m and n each represents a positive integer of from 1 to 2, q represents a positive integer of from 1 to 3, X represents an `acid anion, and Z and Z1 each represents the nonemetallic `atoms necessary -to complete a heterocyclic nucleus selected from the class consisting of a thiazole nucleus, a benzothiazole nucleus, -a naphthothiazole nucleus, -a thianaphtheno-7,6, 4,5-thiazole nucleus, `an oxazole nucleus, =a benzoxazole nucleus, -a naphthoxazole nucleus, :a selenazole nucleus, a benzoselenazole nucleus, a naphthoselenazole nucleus, a thi-azoline nucleus, a Z-quinoline nucleus, a 4-quinoline nucleus, Ia l-isoquinoline nucleus, a 3,3-dialkylindoler1ine nucleus, a 2-pyridine nucleus and a 4-pyridine nucleus, provided that (l) R2 represents la hydrogen latom when q is 3, (2) m and n each represents 1 when Z and Z1, -respectively, represent the atoms necessary to 'complete a heterocyclic nucleus containing 5 'atoms in the heteroc clic ring, and (3) said cyanine dye contains a carboxyl radical on at least one of the groups R, R1, R2, Zand Z1.

References Cited in the iile of this patent UNITED STATES PATENTS OTHER REFERENCES Young et al.: RCA Review, December 1954, pp. 469- 484.

:Nelson: I. Opt. Soc. Am. 46, No. 1, Jaro, 1956, pp. 13- 16 C.A., vol. 43 (1949), 7349d. (Copy in Sci. Lib.) 

1. A PHOTOCONDUCTIVE COMPOSITION COMPRISING PHOTOCONDUCTIVE ZINC OXIDE, A HIGH DIELECTRIC INSULATOR BINDER FOR SAID ZINC OXIDE, AND ADSORBED TO THE SURFACE OF SAID ZINC OXIDE, A CYANINE DYE CONTAINING A CARBOXYL RADICAL ATTACHED TO A CARBON ATOM THEREOF. 